Plants are continually attacked by a diverse range of phytopathogenic organisms. These organisms cause substantial losses to crops each year. Traditional approaches for control of plant diseases have been the use of chemical treatment and the construction of interspecific hybrids between resistant crops and their wild-type relatives as sources of resistant germplasm. However, environmental and economic concerns make chemical pesticides undesirable, while traditional interspecific breeding is inefficient and often cannot eliminate the undesired traits of the wild species. Thus, the discovery of pest and pathogen-resistant genes provides a new approach to control plant disease.
Nematode infection is a significant problem in the farming of many agriculturally significant crops. For example, soybean cyst nematode (Heterodera glycines, herein referred to as “SCN”) is a widespread pest that causes substantial damage to soybeans every year. Such damage is the result of the stunting of the soybean plant caused by the cyst nematode. The stunted plants have smaller root systems, show symptoms of mineral deficiencies in their leaves, and wilt easily. SCN is believed to be responsible for yield losses in soybeans that are estimated to be in excess of $500 million per year. Other pathogenic nematodes of significance to agriculture include the potato cyst nematodes, Globodera rostochiensis and Globodera pallida, which are key pests of the potato, while the beet cyst nematode Heterodera schachtii is a major problem for sugar beet growers in Europe and the United States.
The primary method of controlling nematodes has been through the application of highly toxic chemical compounds. The widespread use of chemical compounds poses many problems with regard to the environment because of the non-selectivity of the compounds and the development of pest resistance to the chemicals. Nematicides such as Aldicarb™, carbamate pesticide, and its breakdown products are known to be highly toxic to mammals. As a result, government restrictions have been imposed on the use of these chemicals. Thus, there is a great need for effective, non-chemical methods and compositions for nematode control.
Various approaches to pest control have been tried including the use of biological organisms which are typically “natural predators” of the species sought to be controlled. Such predators may include other insects, fungi, and bacteria such as Bacillus thuringiensis. Alternatively, large colonies of insect pests have been raised in captivity, sterilized and released into the environment in the hope that mating between the sterilized insects and fecund wild insects will decrease the insect population. While these approaches have had some success, they entail considerable expense and present several major difficulties. For example, it is difficult both to apply biological organisms to large areas and to cause such living organisms to remain in the treated area or on the treated plant species for an extended time. Predator insects can migrate and fungi or bacteria can be washed off of a plant or removed from a treated area by rain. Consequently, while the use of such biological controls has desirable characteristics and has met with some success, in practice these methods have not achieved the goal of controlling nematode damage to crops.
Advances in biotechnology in the last two decades have presented new opportunities for pest control through genetic engineering. In particular, advances in plant genetics coupled with the identification of insect growth factors and naturally-occurring plant defensive compounds or agents offer the opportunity to create transgenic crop plants capable of producing such defensive agents and thereby protect the plants against insect attack and resulting plant disease.
Transgenic plants that are resistant to specific insect pests have been produced using genes encoding Bacillus thuringiensis (Bt) endotoxins or plant protease inhibitors (PIs). Transgenic plants containing Bt endotoxin genes have been shown to be effective for the control of some insects. Effective plant protection using transgenically inserted PI genetic material has not yet been demonstrated in the field. While cultivars expressing Bt genes may presently exhibit resistance to some insect pests, resistance based on the expression of a single gene might eventually be lost due to the evolution of Bt resistance in the insects. Thus, the search for additional genes which can be inserted into plants to provide protection from insect pests is needed.
Additional obstacles to pest control are posed by certain pests. For example, it is known that certain nematodes, such as SCN, can inhibit certain plant gene expression at the nematode feeding site. Thus, in implementing a transgenic approach to pest control, an important factor is to increase the expression of desirable genes in response to pathogen attack. Consequently, there is a continued need for the controlled expression of genes deleterious to pests in response to plant damage.
One promising method for nematode control is the production of transgenic plants that are resistant to nematode infections. For example, with the use of nematode-inducible promoters, plants can be genetically altered to express nematicidal proteins in response to exposure to nematodes. See, for example, U.S. Pat. No. 6,252,138, herein incorporated by reference. Alternatively, some methods use a combination of both nematode-inducible and nematode-repressible promoters to obtain nematode resistance. Thus, WO 92/21757, herein incorporated by reference, discusses the use of a two-promoter system for disrupting nematode feeding sites where one nematode-inducible promoter drives expression of a toxic product that kills the plant cells at the feeding site while the other nematode-repressible promoter drives expression of a gene product that inactivates the toxic product of the first promoter under circumstances in which nematodes are not present, thereby allowing for tighter control of the deleterious effects of the toxic product on plant tissue. Similarly, with the use of proteins having a deleterious effect on nematodes, plants can be genetically altered to express such deleterious proteins in response to nematode attack.
Although these methods have potential for the treatment of nematode infections, their effectiveness is heavily dependent upon the characteristics of the nematode-inducible or nematode-repressible promoters discussed above, as well as, the deleterious properties of the proteins thereby expressed. Thus, such factors as the strength of such nematode-responsive promoters, degree of induction or repression, tissue specificity, or the like can all alter the effectiveness of these disease resistance methods. Similarly, the degree of toxicity and specificity of a gene product to nematodes, the protein's longevity after consumption by the nematode, or the like can alter the degree to which the protein is useful in controlling nematodes. Consequently, there is a continued need for the identification of nematode-responsive promoters and nematode-control genes for use in promoting nematode resistance.